This invention pertains to the field of digital printing, and more particularly to a method of compensating for ink drop volume variation in an inkjet printhead.
An ink jet printer produces images on a receiver by ejecting ink droplets onto the receiver in a raster scanning fashion. The advantages of non-impact, low noise, low energy use, and low cost operation in addition to the capability of the printer to print on plain paper are largely responsible for the wide acceptance of ink jet printers in the marketplace.
A typical inkjet printer uses one printhead for each color of ink, where each printhead contains an array of individual nozzles for ejecting drops of ink onto the page. The nozzles are typically activated to produce ink drops on demand at the control of a host computer, which processes raster image data and sends it to the printer through a cable connection. It is known to those skilled in the art that undesirable image artifacts can arise due to small differences between the individual nozzles in a printhead. These differences, often caused by slight variations in the manufacturing process, can cause the ink drops ejected from one nozzle to follow a trajectory that is slightly different from neighboring nozzles. Also, each nozzle may produce ink drops that are slightly different in volume from neighboring nozzles. Larger ink drops will result in darker (increased optical density) areas on the printed page, and smaller ink drops will result in lighter (decreased optical density) areas. Due to the raster scanning fashion of the printhead, these dark and light areas will form lines of darker and lighter density often referred to as xe2x80x9cbandingxe2x80x9d, which is generally quite undesirable and results in a poor quality print.
There are many techniques present in the prior art that describe methods of reducing banding artifacts caused by nozzle-to-nozzle differences using methods referred to as xe2x80x9cinterlacingxe2x80x9d, xe2x80x9cprint maskingxe2x80x9d, or xe2x80x9cmultipass printingxe2x80x9d. These techniques employ methods of advancing the paper by an increment less than the printhead width, so that successive passes or swaths of the printhead overlap. This has the effect that each image raster line may be printed using more than one nozzle, and drop volume or drop trajectory errors observed in a given printed raster line are reduced because the nozzle-to-nozzle differences are averaged out as the number of nozzles used to print each raster line increases. See, for example, U.S. Pat. Nos. 4,967,203 and 5,992,962. Other methods known in the art take advantage of multipass printing to reduce banding by using operative nozzles to compensate for failed or malperforming nozzles. For example, U.S. Pat. Nos. 6,354,689 and 6,273,542 to Couwenhoven et al., teach methods of correcting for malperforming nozzles that have trajectory or drop volume errors in a multipass inkjet printer wherein other nozzles that print along substantially the same raster line as the malperforming nozzle are used instead of the malperforming nozzle. However, the above mentioned methods provide for reduced banding artifacts at the cost of increased print time, since the effective number of nozzles in the printhead is reduced by a factor equal to the number of print passes. Also, many of the prior art techniques described above rely on the performance of the individual ink nozzles being fairly uncorrelated. In other words, if four different nozzles are used to print a given raster line, then the banding artifacts will be reduced only if those four nozzles had different drop volume characteristics. If all four of those nozzles happen to eject ink drops that were larger than average, then an improvement in banding will not be observed, and a significant penalty will be paid in terms of increased print time. Such instances can occur if the-nozzle-to-nozzle variation changes slowly across the printhead.
Other techniques known in the art attempt to correct for drop volume variation by modifying the electrical signals that are used to activate the individual nozzles. For example, U.S. Pat. No. 6,428,134 to Clark et al., teaches a method of constructing waveforms for driving a piezoelectric inkjet printhead to reduce ink drop volume variability. Similarly, U.S. Pat. No. 6,312,078 to Wen et al. teaches a method of reducing ink drop volume variability by modifying the drive voltage used to activate the nozzle.
Still other techniques known in the prior art address drop volume variability issues between printheads. For example, U.S. Pat. No. 6,154,227 to Lund teaches a method of adjusting the number of microdrops printed in response to a drop volume parameter stored in programmable memory on the printhead cartridge. This method reduces print density variation from printhead to printhead, but does not address print density variation from nozzle to nozzle within a printhead. U.S. Pat. No. 5,812,156 to Bullock et al., teaches a method of using drop volume information to determine ink usage in an inkjet printhead cartridge, and warn the user when the cartridge is running low on ink. This method includes storing ink drop volume information in programmable memory on the cartridge, but does not teach characterizing the drop volume produced by individual nozzles, nor how that information may he used to correct image artifacts. Also, U.S. Pat. Nos. 6,450,608 and 6,315,383 to Sarmast et al., teach methods of detecting inkjet nozzle trajectory errors and drop volume using a two-dimensional array of individual detectors.
The inkjet printing market continues to require faster and faster printing of images, and many modifications to the basic inkjet printing engine have been investigated to accommodate this requirement. One method of printing an image faster is to use a printhead that has more nozzles. This prints more image raster lines in each movement of the printhead, thereby increasing the throughput of the printer. However, manufacturing and technical challenges prevent the creation of printheads with large numbers of nozzles. Thus, in some state of the art inkjet printers designed for high throughput, several smaller printheads have been assembled into a single printhead xe2x80x9cmodulexe2x80x9d that effectively increases the number of nozzles, but uses smaller printheads that are easier to manufacture. In this arrangement, it is not uncommon for the above described image artifacts associated with drop volume variation to become amplified. This is due to the fact that combining several smaller printheads into a single larger module often results in slowly varying nozzle-to-nozzle differences, which the prior art techniques are ill-equipped to handle.
Thus, there is a need for a method of reducing image artifacts associated with slowly varying nozzle-to-nozzle variability, while simultaneously maintaining high image quality and short print times.
It is an object of the present invention to provide for printing high quality digital images that are free of the above-described artifacts associated with slowly varying nozzle-to-nozzle variability.
This object is achieved by a method for modifying a digital image having an array of raster lines, each raster line having an array of image pixels, to produce a modified digital image suitable for printing on an inkjet printer containing at least one printhead having nozzles, such that unwanted optical density variations in the print are reduced, comprising:
a) determining an optical density parameter for each nozzle in the printhead;
b) determining a line correction factor for a given raster line in response to the optical density parameter for each nozzle in the printhead and the raster line number; and
c) modifying each pixel in the given raster line in response to the line correction factor to produce the modified digital image.
The present invention has an advantage in that it provides for a method of reducing undesirable banding artifacts in an image printed with a printhead that has slowly varying nozzle-to-nozzle variability.
Another advantage of the present invention is that it provides for short printing times by reducing the number of banding passes required to achieve high print quality.
Yet another advantage of the present invention is that a high quality print is achievable with a previously unacceptable printhead. This increases the manufacturing yield of acceptable printheads from the factory.